A circuit board used for mounting various electronic components, e.g., a semiconductor element, a heater element, or a Peltier element, is provided with a metal wiring layer on at least one principal surface of a ceramic sintered body. An electronic apparatus in which an electronic component is mounted on the metal wiring layer in the circuit board with an electrode pad or the like therebetween has been used in various apparatuses.
Meanwhile, with respect to such a circuit board, for the purpose of suppressing peeling of the metal wiring layer from the ceramic sintered body because of the heat generated during operation of the electronic component mounted on the circuit board, development of a material for a paste serving as the metal wiring layer and studies on a method for forming the metal wiring layer have been performed to enhance the adhesion strength to the ceramic sintered body.
For example, PTL 1 proposes a method for manufacturing a thick film circuit board by using a copper conductor paste. In the method for manufacturing a thick film circuit board by printing the copper conductor paste on a substrate and forming a copper film through firing, a first layer is formed by printing the copper conductor paste containing glass frit on a ceramic substrate and performing drying and, subsequently, at least one layer is formed thereon by printing the copper conductor paste not containing glass frit and performing drying, followed by firing to form a thick copper film.